1. Field of the Invention
The present invention relates to a mist spraying apparatus and image forming apparatus, and more particularly, to a mist spraying apparatus which sprays a liquid in the form of a mist, and an image forming apparatus which forms images on a recording medium, such as a paper, by means of a mist spray.
2. Description of the Related Art
An ink mist type of image forming apparatus which forms images by generating an ink mist (very fine ink particles) by means of ultrasonic vibration, and depositing this ink mist onto a recording medium, such as paper (see, for example, Japanese Patent Application Publication No. 5-57891 (and in particular, FIG. 3) and “Study on Ink Ejection of Print Head Using Focused Ultrasonic Wave and Nozzle” (Shumpei Kameyama, Hiroshi Fukumoto, and Shusou Wadaka, Journal of the Acoustical Society of Japan, Vol. 60, No. 2, (2004), pp. 53-60)).
Generally, a piezoelectric body is used as a device for generating ultrasonic vibration. For example, a piezoelectric body made of lead zirconate titanate (PZT) which has been polarized previously in the thickness direction is used, and a voltage having a frequency corresponding to an ultrasonic wave (for example, 10 MHz) is applied in the thickness direction of the piezoelectric body. By utilizing the displacement (distortion) generated in the thickness direction of the piezoelectric body, when a diaphragm is caused to vibrate and an ultrasonic wave is applied to the ink inside an ink chamber, the meniscus at the nozzle connected to the ink chamber becomes unstable, and ink in the form of a mist is sprayed from the nozzle.
However, in an ink mist type image forming apparatus using a conventional piezoelectric body, it is difficult to achieve a mist spray of ink having a high viscosity (for example, approximately 20 cP).
For example, the amplitude of movement of a diaphragm in the d33 mode (where the polarization direction, electric field direction and direction of distortion are all the same) in the case of a piezoelectric body having a thickness of several tens micrometers (μm) to several hundred micrometers, is approximately ten nanometers (nm) when driven at several tens volts (V). Even if the piezoelectric body has a laminated structure, the amplitude of the piezoelectric body is currently limited to the sub-micron order.
On the other hand, when spraying ink in the form of a mist, the amplitude required in order to destabilize the meniscus surface is directly proportional to the viscosity of the ink that is to be sprayed. For example, the meniscus of ink having a viscosity of several centipoises (cP) can be broken down at an amplitude of the order of several hundred nanometers, but the meniscus of ink having a viscosity of several tens centipoises cannot be broken down unless the amplitude reaches several micrometers.
It is possible to multiply the amplitude by several times, by attaching horns of various shapes. However, even if the amplitude is increased in this way, in a conventional image forming apparatus which uses piezoelectric bodies made of PZT, or the like, it is not possible to break down the meniscus of ink having a viscosity of around 20 cP, and therefore, ink of this viscosity cannot be sprayed in the form of a mist. For example, with a single-plate piezoelectric body, it is not possible to achieve meniscus break-down in ink having a viscosity of 10 cP.